The invention relates to a method and a circuit arrangement for intermittent application of an electrical DC or AC voltage by means of a switch element to an electrode which is in contact with a material or substance. The material can lead to soiling of the electrode surface. The dirt layer forming on the electrode surface can electrically insulate the electrodes in such a manner that their functionability is impaired.
An electrical cleaning method is known in which to the electrodes a DC or AC voltage of adequate magnitude is intermittently applied and this leads to the cleaning thereof. The intermittent application of the cleaning voltage is effected with switches (e.g. relays) which connect the electrode to a correspondingly dimensioned voltage source. However, with this additional wiring the current or voltage level carried by the electrode is in particular capacitively loaded. This is troublesome in particular in magnetic-inductive flow measurement in which electrodes contacting a flow medium generate a measuring signal, for the electrode represents a voltage source with internal resistance which depends in particular on the geometry of the magnetic-inductive pickup and the conductivity of the medium. The higher the value of this internal resistance the more sensitive the electrode becomes as signal source to external circuitry. In addition, switches can generate interfering voltages which have undesirable effects on the electrode signal. Adequate magnitude of voltage is that amount of voltage necessary for cleaning off the electrodes by the aforementioned known method so as to peel or scale off deposits or decompose contaminants. With a serial resistor of 4700.OMEGA. to the electrode, a diameter of 8 mm, and an area of 50 mm.sup.2, a cleaning voltage of +5 volts to +15 volts would normally provide the necessary cleaning function to remove conductive sheets covering the electrode and liner. A voltage of -5 volts to -15 volts would normally be used to prevent oxidation of an electrode such as a tantalum electrode. The voltage pulse should be applied for about 700 ms every 10 minutes. The voltage pulse duration can be varied depending upon the degree of soiling of the electrodes.
The invention is thus based on the problem in applying a voltage in the manner set forth at the beginning of freeing the electrode and/or the connecting/signal lines from interfering influences originating from the switch for applying the voltage.
A method according to the invention for solving this problem resides in that--at least during the period in which no voltage is applied--the electrical potential of the electrode is coupled approximately to the terminal or terminals of the switch element which are not allocated to the electrode. A terminal of this switch not connected to the electrode or the line thereof is operated at an electrical potential which at least approximately corresponds to the potential of the electrode and/or the line thereof. As a result, in the disconnected (interrupting the connection to the cleaning voltage) state of the switch potential differences between the terminal for example allocated to the cleaning voltage and the terminal allocated to the electrode are held negligibly small. Consequently, inevitable capacitive components of the switch can no longer inadmissibly load the electrode signal. The means for coupling the electrode potential to the switch are expediently implemented with active components, for example operational amplifiers. This gives the advantage that any interfering voltages originating from the switch element can be substantially compensated by the output of the active component.
The method according to the invention can be advantageously used in cases where electrodes are in contact with flowing medium for measuring purposes and thereby become soiled. This is the case in particular in magnetic-inductive flow measurement where the electrodes serve to generate a measuring signal. In these cases said method can be incorporated as follows into a measuring cycle:
a measuring signal originating from the electrodes is stored prior to the application of the cleaning voltage and output as measured value; PA1 the cleaning voltage is applied for a predetermined selectable duration and in predetermined selectable intervals t the electrodes; PA1 after the disconnection of the cleaning voltage restarting of the measuring operation is delayed by a period in which PA1 the electrochemical conditions at the cleaned electrode can become normal; and PA1 after said period the measuring operation is restarted.
In a circuit designed for the method according to the invention an impedance converter with high input and low output impedance is provided, the output of which is connected to a terminal of the switch not connected to the electrode. The input of the impedance converter can either be directly connected to the electrode or the connecting signal line thereof or can be brought approximately to the electrode potential by means of separate coupling members (e.g. operational amplifier circuits).
According to an embodiment of the invention the DC or AC voltage source for generating the cleaning voltage is connected to the input of the impedance converter, parallel to the supply line of the electrode potential. Between the output of the voltage source and the input of the impedance converter switch means may be provided which are intermittently driven by a time control.
According to an alternative embodiment a terminal of the switch not connected to the electrode or the line thereof is allocated both to the output of the voltage source and to the output of the impedance converter. This can be effected on the one hand by a changeover switch which switches the output of the cleaning voltage source to and fro intermittently between a terminal to the electronics and the output of the impedance converter. On the other hand, the output of the impedance converter can permanently contact a terminal of the switch not connected to the electrode, the cleaning voltage being intermittently switched through to the output of the impedance converter. According to a further development of the invention the voltage source for generating the cleaning voltage and the impedance converter are integrated in their circuitry to form a unit. This can for example be done by an operational amplifier which is connected with a feedback resistance network and in which at least one resistance value influencing the gain is intermittently variable. In such a case, during the time phase in which no cleaning takes place, the operational amplifier is operated as impedance converter with the gain 1; during the cleaning phase the gain is increased to a greater value corresponding to the necessary cleaning voltage by influencing said resistance. A particularly simple and economical embodiment of the circuit arrangement according to the invention resides in that the switch for the application of the cleaning voltage is implemented by one or more diodes. If AC voltage is employed for the electrode cleaning a diode circuit is expedient in which two diodes are arranged antiparallel to each other. A further advantage resides in that the impedance converter can be additionally used as potential driver for any electrode shields or other shields.
The circuit arrangement according to the invention is particularly suitable for use in magnetic-inductive flow meters or in measuring material monitoring devices in which in each case electrodes in contact with medium are employed for generating a measuring signal.